Thin interbed analysis has guiding significance for oil and gas exploration, and it is feasible to precisely predict their thickness variation through the time-frequency (TF) features of seismic signals. In this letter, an auto-reassigning transform (ART) is proposed to analyze seismic thin interbeds. This ART retrieves the instantaneous frequency (IF) trajectory by local maxima, then determines the TF coefficients within the effective window by the $3\sigma$ criterion, and finally reassigns these coefficients automatically to the retrieved IF trajectory. Therefore, such ARTs provide a more concentrated TF representation (TFR) with reconstruction capability. Synthetic signals and seismic data are employed to demonstrate the effectiveness of the proposed ART by comparing it with state-of-the-art TF analysis methods.